fixing typo [ci skip]

per report of Vinicius Segalin
Fix pgvm install for OS X

There is some differences in usage of GNU sed and BSD sed.
Fix pgvm use for OS X

* Use mkdir -p instead of mkdir --parents
* Use rm instead of forcing the current symlink
* Use ln -s instead of ln --no-dereference --symbolic
Fix pgvm cluster create for OS X

* Use mkdir -p instead of mkdir --parents